Method for producing a R-Ce-Cu-O superconducting material wherein R is at least one rare earth element

ABSTRACT

The disclosed superconducting oxide material has a crystal structure of either Nd 2  CuO 4  type or oxygen-deficient perovskite type and mainly consists of a composition having a general chemical formula of (R 1-x  A x ) m+1  Cu m  O 3m+1-y , R being at least one rare earth element selected from the group consisting of Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Er, Tm, Yb, and Lu, A being Ce or Th, m being an integer of 1, 2, 3, . . . ∞ (m=∞ standing for (R 1-x  A x )CuO 3-y ), 0≦x≦1, 0≦y≦m, Cu in the material having an average valence of not larger than +2 for all R, A, m, x, and y. 
     The disclosed superconducting oxide material may have a general chemical formula of Nd 2-x-z  Ce x  Sr z  CuO 4-y , 0≦x≦1, 0≦y≦1, and 0≦z≦1. 
     The above superconducting oxide material can be made by mixing powdery starting materials for the composition, sintering the mixture at 1,000° to 1,100° C. in air, and heat treating the sintered mixture at 900° to 1,100° C. in a reducing atmosphere (with an oxygen partial pressure of less than 10 -2  atm.)

This is a divisional of application Ser. No. 07/470,011 filed Jan. 25,1990 now U.S. Pat. No. 5,143,895.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to superconducting oxide material whose electricresistance diminishes at a low temperature and a method for producingthe same. The superconducting oxide material of the inventionsuperconducts at a comparatively high temperature, e.g., the liquefyingtemperature of hydrogen or -253° C. (20 K.), and it can be produced in areducing atmosphere.

An object of the invention is to provide a material which is suitablefor electronics elements, such as SQUIDs (Superconducting QuantumInterference Devices) and superconducting transistors, superconductingwires, and the like.

2. Related Art Statement

The occurrence of phenomenon of superconductivity has been known only ata very low temperature of 4-23 K., and its application has been limitedto those fields where use of such very low temperature is economicallyjustifiable. In 1986, rare each element-copper-oxide material wasinvented to make a great jump in the improvement of the transitiontemperature for superconductivity, and superconductivity is nowavailable at a more readily accessible low temperature, e.g., theliquefying temperature of air.

Process which has been developed so far as the preparation of suchsuperconducting oxide material uses a number of heat treating steps inan oxidizing atmosphere so as to introduce oxygen into the material. Inproducing electronics devices, thin film techniques under high vacuum isfrequently used, and the need of heat treatment in the oxidizingatmosphere has been a major difficulty to be overcome before realizingthe application of the superconducting oxide material to electronics.Further, in the case of superconducting wires for electric powertransmission and storage, sheaths made of oxidization resisting noblemetal, such as silver or platinum, have been used, and the use of costlynoble metal has hampered popular application of the superconductingoxide material to power industries.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to overcome theabove-mentioned problems of the conventional oxide superconductors, byproviding an improved superconducting oxide material which can beproduced in vacuo or in a reducing atmosphere.

Another object of the invention is to develop n-type superconductingoxide material whose current carriers are electrons. Conventional oxidesuperconductors containing copper as a main ingredient are of p-type,and their current carriers are positive holes (i.e., holes left afterrelease of electrons).

A superconducting oxide material according to the invention is ofcrystal structure of either Nd₂ CuO₄ type or oxygen-deficient perovskitetype, and mainly consists of a composition having a general chemicalformula of (R_(1-x) A_(x))_(m+1) Cu_(m) O_(3m+1-y).

In the above general chemical formula, R is at least one rare earthelement selected from the group consisting of Pr, Nd, Sm, Eu, Gd, Dy,Tb, Ho, Er, Tm, Yb, and Lu or a mixture thereof at an arbitrary ratio.

The symbol A in the above general chemical formula is either Ce or Th inthe main, and it may contain one or more alkaline earth metals such asCa, Sr, and Ba. The constant m being an integer of 1, 2, 3, . . . ∞ (m=∞standing for (R_(1-x) A_(x))CuO_(3-y)).

The constant x and y are in the ranges of 0≦x≦1, 0≦y≦m, preferably0≦x≦0.2, 0≦y≦0.2.

For all combinations of elements represented by the symbols R and A, forall values of the constant m, and for all the above ranges of constantsx and y, Cu in the material has an average valence of not larger than+2.

It is another object of the invention to provide a superconducting oxidematerial with a crystal structure of Nd₂ CuO₄ type, or oxygen-deficientperovskite type, which material is synthesized in a reducing atmosphereand mainly consists of a composition having a general chemical formulaof Nd_(2-x-z) Ce_(x) Sr_(z) CuO_(4-y), 0≦x≦1, 0≦y≦1, and 0≦z≦1.

It is a further object of the invention to provide a method forproducing a superconducting oxide material mainly consisting of acomposition with a general chemical formula of (R_(1-x) A_(x))_(m+1)Cu_(m) O_(3m+1-y), R being at least one rare earth element selected fromthe group consisting of Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Er, Tm, Yb, andLu, A being Ce or Th, m being an integer of 1, 2, 3, . . . ∞ (m=∞standing for (R_(1-x) A_(x))CuO_(3-y)), 0≦x≦1, 0≦y≦m, Cu in the materialhaving an average valence of not larger than +2 for all R, A, m, x, andy, which method comprises steps of mixing powdery starting materials forthe composition, sintering the mixture at 1,000° to 1,100° C. in air,and heat treating the sintered mixture at 900° to 1,100° C. in areducing atmosphere (with an oxygen partial pressure of less than 10⁻ 2atom.), so as to make crystal structure of the material into either Nd₂CuO₄ type or oxygen-deficient perovskite type.

In the present invention the phrase "mainly consisting of" means thatthe above-mentioned composition constitutes the major portion of thesubject material and leaves the subject material open for the inclusionof such metal or composition which is different from the above-mentionedcomposition, provided that such inclusion does not cause any detrimentaleffects on the crystal structure and the purposes of the invention.

The material of the invention mainly consisting of the above-mentionedcomposition is an oxide and yet maintains good superconductivity evenwhen its oxygen content is reduced unless its crystal structure isbroken. Rather, reduction of oxygen content can improve thesuperconducting characteristics of the material as shown in the ensuingembodiments. Thus, the material of the invention is different fromconventional oxide superconductor in that the superconducting materialcan be formed in an atmosphere with a low oxygen partial pressure oreven in vacuo.

Important factors in the superconducting oxide material of the inventionare as follows.

(1) The superconducting oxide material is a composition with a generalchemical formula of (R_(1-x) A_(x))_(m+1) Cu_(m) O_(3m+1-y), and thematerial has crystal structure of either Nd₂ CuO₄ type oroxygen-deficient perovskite type.

(2) In the above general chemical formula, R is at least one rare earthelement selected from the group consisting of Pr, Nd, Sm, Eu, Gd, Dy,Tb, Ho, Er, Tm, Yb, and Lu, and preferably from the group consisting ofPr, Nd, Sm, Eu and Gd.

(3) In the above general chemical formula, A is either Ce or Th, and maycontain at least one of alkaline earth metal of Ca, Sr, and/or Ba.

(4) In the above general chemical formula, m is an integer of 1, 2, 3, .. . ∞.

(5) In the above general chemical formula, x and y are in the ranges of0≦x≦1, 0≦y≦m, preferably 0≦x≦0.2, 0≦y≦0.2.

(6) For all combinations of elements represented by the symbols R and A,for all values of m, and for all ranges of constants x and y, Cu in thematerial has an average valence of not larger than +2.

Examples of what is covered by the above general chemical formula are asfollows.

    (R.sub.1-x A.sub.x).sub.m+1 Cu.sub.m O.sub.3m+1-y          (i)

    Nd.sub.2-x-z Ce.sub.x Sr.sub.z CuO.sub.4-y                 (ii)

(for the case of R=Nd, A=Ce and Sr in formula (i))

    Nd.sub.2-x Ce.sub.x CuO.sub.4-y                            (iii)

(for z=0 in formula (ii))

    Nd.sub.2 CuO.sub.4-y                                       (iv)

(for x=0 in formula (iii))

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to theaccompanying drawing, in which:

FIGS. 1A, 1B, and 1C are diagrammatic illustrations of the crystalstructures in the T' phase, T phase, and T* phase, respectively. The T'phase, so called Nd₂ CuO₄ type structure is the superconducting oxidematerial of the invention,

FIG. 2 shows temperature versus resistivity curves, indicating presenceand absence of superconductivity in the material of the invention forthe case of being quenched in air and for the case of being treatedunder reduced pressure;

FIGS. 3A, 3B, and 3C are x-ray intensity curves obtained in x-raydiffraction tests on specimens of superconducting materials withstructures shown in FIGS. 1A, 1B and 1C, respectively; and

FIG. 4 is Meissner magnetization characteristics diagram of specimens ofthe superconducting oxide material of the invention (Ln₁.85 Ce₀.15CuO_(4-y), Ln being Pr, Nd, or Sm) for a certain range of absolutetemperature T(K).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Ever since the discovery of high-temperature superconductivity ofcuprate by George Bendnorz and Alex Muller, a large number of studieshave been carried out to find out and clarify difficulties in obtaininga high critical transition temperature Tc (to be referred to as high-Tc,hereinafter). Prior studies in this field have found that there areseveral families of high-Tc cuprate, and such families have certaincommon characteristics; namely, laminar structure with two-dimensionalarrays of octahedra or CuO pyramids which are doped with holes. Theinventors have discovered a new superconducting cuprate doped withelectrons (not doped with holes), which is a kind of breakthrough of theestablished chemical and physical theories on solid phase high-Tccuprate.

The new superconducting material is a compound doped with, for instance,Ce⁴⁺ and its general chemical formula is Ln_(2-x) Ce_(x) CuO_(4-y), Lnbeing Pr, Nd, or Sm. This compound shows a T' phase structure, as shownin FIG. 1A for the case of La_(2-x) Sr_(x) CuO₄ (Tc=40 K.), and a T*phase structure, as shown in FIG. 1B for the case of Nd_(2-x-z) Ce_(x)Sr_(z) CuO₄ (Tc=30 K.).

The so-called T' phase structure of Nd₂ CuO₄ type in FIG. 1A consists oftwo-dimensional plane of Cu-O. As compared with the conventionalsuperconducting copper oxide, this structure is characterized by lack ofapex oxygen atoms.

Specimens of the material of the invention were prepared from a startingmaterial which was a powdery mixture of CuO with compounds of rare earthelements such as those selected from the group of CeO₂, Pr₆ O₁₁, Nb₂ O₃,and Sm₂ O₃. The powdery mixture was at first calcined at 950° C. for 10hours in air, and shaped into pellets under pressure, and finallysintered at 1150° C. for 12 hours in air. The sintered pellets werecooled quickly in air to room temperature. To make the specimensuperconducting, the pellets were doped with Ce in a reducing atmosphereand then annealed. In particular, the pellets were annealed at about1,000° C. for 10 hours in a flow of Ar/O₂ gaseous mixture with an oxygenpartial pressure of less than 10⁻³ atom, and cooled quickly in the sameatmosphere to room temperature. The best result was obtained when theoxygen partial pressure was low, e.g., 8×10⁻⁵ atom.

FIG. 2 shows the temperature dependence of the resistance of Nd_(2-x)Ce_(x) CuO_(4-y). The compound Nd₂ CuO₄ before doping is a typicalsemiconductor, and doping the specimen with Ce⁴⁺ makes it conductive.When the specimen was made oxidation-deficient (y being less than 0.04)by quickly cooling non-doped specimen from 1,150° C. to roomtemperature, considerable reduction of resistance was noticed.Observation of such fact suggests that the doped Ce⁺⁴ and/or oxygendeficiency clearly indicates that electrons are introduced into thecompound as charge carriers. According to recent studies, T' phasestructure of Nd-based copper oxide cannot be doped with holes.

The T* phase structure of the compound Nd_(2-x-z) Ce_(x) Sr_(z) CuO₄ asshown in FIG. 1C can be doped with holes like the T phase structure ofFIG. 1B, but it cannot be doped with electrons. With the specimen ofNd_(2-x) Ce_(x) CuO_(4-y), when the Ce concentration was 0.15, thespecimen was semi-metallic and not superconducting, and a very lowresistivity (less than 10⁻² Ω.cm) was noted.

In FIG. 2, the specimen with x=0.15 showed a resistivity drop at about 9K. This specimen shows superconductivity in small portions. In the samespecimen with x=0.15, a remarkable superconductivity was acheived by theabove-mentioned reduction process. Variation of resistance in responseto magnetization indicated the start of becoming superconductive atabout 24 K. The q number, which corresponds to electron concentrationintroduced per unit [Cu-O] or the effective Cu valence 2-q, wasevaluated by iodomentry while considering reducing reaction between Ce³⁺and Ce⁴⁺ in the titrating process. The specimen with x=0.15 in FIG. 2showed the q value of 0.20 before the reducing treatment, and the qvalue of the superconducting specimen increased up to 0.28. Thiscorresponds to the valence increment of oxygen at less than y=0.07.Thus, the increase of electron in excess of a certain value of density(q being less than 0.20) can be considered as an increase ofsuperconductivity.

Electron type carriers in the new superconducting compound was confirmedby the measurement of Hall coefficient R_(H).

A specimen Nd₁.85 Ce₀.15 CuO₀.93 with Tc at 24 K. showed negative Hallcoefficients; namely,

at 300 K., R_(H) =-6.5×10⁻⁴ cm³ /c

at 80 K., R_(H) =-2.3×10⁻³ cm³ /c

Such negative Hall coefficients provide contrasty difference of thematerial of the invention from conventional superconducting cuprate ofhole type whose Hall coefficients are always positive.

FIG. 3A shows the x-ray diffraction pattern of the specimen Nd₁.85Ce₀.15 CuO₃.93. For comparison, similar x-ray diffraction pattern of theT phase structure of the hole type superconducting specimen La₁.85Sr₀.15 CuO₄ of FIG. 1B is shown in FIG. 3B, and the x-ray diffractionpattern of the T* phase structure of the specimen Nd₁.4 Ce₀.2 Sr₀.4 CuO₄of FIG. 1C is shown in FIG. 3C.

Essentially, the x-ray diffraction pattern of the specimen Nd₁.85 Ce₀.15CuO₀.93 of FIG. 3A coincides with the x-ray diffraction pattern of theT' structure of the compound before the doping of Ce. More specifically,all the recognizable diffraction peaks in FIG. 3A can be correlated toabout the same lattice constants as those of T' phase structure (a=3.93Å, c=12.11 Å) belonging to the typical tetragonal system having a=3.95 Åand c=12.07 Å. In the T phase structure of FIG. 3B, a=3.78 Å and c=13.2Å, while in the T* phase structure of FIG. 3C, a=3.85 Å and c=12.5 Å. Ascompared with the lattice parameters of the T phase and T* phasestructures having apex oxygen atoms bonded thereto, the T' phasestructure has an expanded Cu-O plane and a shrunk axis thereof.

It should be emphasized here that the electron type superconductivity isnot limited to Nd compounds alone. In the case of T' phase structures ofother compounds such as cuprates of Pr⁻ and Sm⁻, the electronconcentration may be increased in excess of 0.20 by Ce doping or byreducing treatment so as to expand the superconducting range to 20 K.

FIG. 4 shows the results of Meissner effect measurement on threedifferent specimens of Ln₁.85 Ce₀.15 CuO₄ (Ln being Pr, Nd, and Sm,respectively) in a magnetic field of 10 Oe (while cooling in themagnetic field) by using a SQUID magnet meter. Meissner signals ofconsiderable magnitude were measured in the series of the specimens.

In particular, Nd₁.85 Ce₀.15 CuO₃.93 produced a large signal (more than25% of the theoretical value) which was related to a largesuperconductivity with a Tc=24 K. As to the measured value of Tc, theywere similar to the Tc of hole type superconducting cuprate of singlelayer; namely, Tc=28 K. for T* phase structure of CuO pyramids, andTc=40 K. for T phase structure of CuO octahedra.

The Pr₁.85 Ce₀.15 CuO_(4-y) compound specimen is the first example ofPr-cuprate. Of the hole-doped cuprates, Pr-cuprates such as the T* phasecompound Pr_(2-x-y) Ce_(x) Sr_(z) CuO_(4-y) and the so-called 1-2-3compound PrBa₂ Cu₃ O₇ show neither metallic behavior nor superconductingbehavior. This is due to the partial trap of holes caused by the mixedvalence (3⁺, 4⁺) of Pr ions.

The fact that the Pr_(2-x-y) Ce_(x) Sr_(z) CuO_(4-y) compound lackssuperconductivity is another proof of the electron carriers in thesuperconduction of the T' compounds. In other words, Pr³⁺ cannot bereduced further.

In short, the inventors discovered that a sheet of CuO₂ which is dopedwith electrons but free from apex oxygen atoms superconducts with animproved Tc of as high as 24 K. Such discovery provides a new decisiveprinciple in the science of high-Tc superconductors, and it provides animportant tool for analyzing the mechanism of high-Tc.

EMBODIMENTS

The invention will now be described in further detail by referring toembodiments, but the invention is not restricted by such embodiments.

Embodiment 1

Copper-oxygen compositions of the (R_(1-x) A_(x))_(m+1) Cu_(m)O_(3m+1-y) system were prepared while adjusting the ingredientconcentrations in the ranges of m=1, 0≦x≦0.1, 0≦y≦0.1, R=Pr, Nd, Sm, andA=Ce.

More particularly, each of special class reagents Nd₂ O₃, CeO₂, and CuOwere measured and the measured amounts of them were mixed in an agatemortar, and the powdery mixture was placed in a crucible and pre-firedat 950° C. for five hours in air. The pre-fired mixture was pressed intopellets under a pressure of about 1,000 kg/cm², and the pellets weresintered at 1,100° C. for five hours in air. The thus sintered pelletswere heat treated for five hours in an Argon-Oxygen mixed gas (with anoxygen partial pressure of 10⁻³ to 10⁻⁴ atom), and then quenched to roomtemperature in the same atmosphere.

X-ray diffraction tests were carried out on the sintered pellets andthey proved to have the Nd₂ CuO₄ type crystal structure.

Among the specimens thus prepared, those with a composition ofNd:Ce:Cu=1.85:0.15:1.0, i.e., x=0.075, proved to begin the diminishingof electric resistance at 24 K. and to reach complete superconduction at19 K. their diamagnetic magnetization under the complete superconductionreached more than the ideal value of 30%.

EFFECT OF THE INVENTION

The superconducting material of the invention can be applied toelectronics elements, such as SQUIDs and superconducting transistors,superconducting wires, and the like. The material of the invention hasthe following advantages.

(a) The superconducting material can be produced in a reducingatmosphere. This is an important difference from conventional oxidesuperconductors.

(b) The superconducting material can be formed into thin films moreeasily than before thanks to the availability of the reducingatmosphere. Thin films are necessary in producing electronics elements.

(c) need of noble metals in the acid-resisting sheaths ofsuperconducting wires can be eliminated and replaced with the materialof the invention.

(d) The superconducting material can be prepared by using abundantlyavailable rare earth elements such as Ce, Pr, and Nd.

(e) The superconducting material is of n-type, i.e., electrons arecarriers of current. Conventional oxide superconductors are of p-type,i.e., holes are carriers of current. Thus, with the invention, bothp-type and n-type oxide superconductors are available, and gate is nowopened for development of multi-purpose superconducting electronicselements, as in the case of semiconductor materials.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous changes in details may beresorted to without departing from the scope of the invention ashereinafter claimed.

What is claimed is:
 1. A method for producing a superconducting oxidematerial, the material consisting essentially of a composition having ageneral chemical formula of

    (R.sub.1-x Ce.sub.x).sub.m+1 Cu.sub.m O.sub.3m+1-y,

R being at least one rare earth element selected from the groupconsisting of Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Er, Tm, Yb, and Lu, mbeing an integer of 1, 2, 3, . . . ∞ (m=∞ standing for (R_(1-x)A_(x))CuO.sub.(3-y)), 0≦x≦1, 0≦y≦m, and Cu in the material having anaverage valence of not larger than +2 for all R, A, m, x, and y, themethod comprising mixing powdery starting materials for the composition,sintering the mixture at 1,000° to 1,100° C. in air, and heat treatingthe sintered mixture at 900° to 1,100° C. in a reducing atmosphere withan oxygen partial pressure of less than 10⁻² atm., so as to form acrystal structure of either an Nd₂ CuO₄ type or oxygen-deficientperovskite type.